1. Technical Field
The present invention relates to an FFS mode liquid crystal display device which can be manufactured without increasing the number of manufacturing steps, and can achieve a high aperture ratio and high display image quality with a pixel electrode and a common electrode disposed on a planarization film, and a method of manufacturing the same.
2. Related Art
Liquid crystal display devices of a vertical electric field system including a pair of transparent substrates having an electrode and the like formed on the surface, and a liquid crystal layer held between the pair of substrates, in which various pieces of information are displayed by applying voltage to the electrodes on the substrates to realign liquid crystals, have been popularly used. However, a twisted nematic (TN) mode generally used in such liquid crystal display devices of a vertical electric field system has a problem in that the viewing angle is small. Therefore, various liquid crystal display devices using an improved vertical electric field system such as a vertical alignment (VA) mode and a multidomain vertical alignment (MVA) mode have been developed.
Aside from the liquid crystal display devices of a vertical electric field system described above, in-plane switching (IPS) mode or fringe field switching (FFS) mode liquid crystal display devices including a pair of a pixel electrode and a common electrode on only one substrate are also known.
In an IPS mode liquid crystal display device, the pair of electrodes is disposed on a single layer, and liquid crystal molecules are realigned in a direction parallel to the substrate by applying an electric field to liquid crystals in a direction approximately parallel to the substrate. Therefore, IPS mode liquid crystal display devices are also referred to as a liquid crystal display device of a horizontal electric field system, and have the advantage of a significantly large viewing angle compared to the liquid crystal display of a vertical electric field system described above. However, IPS mode liquid crystal display devices have the problem that the liquid crystal molecules above the pixel electrode are not sufficiently driven due to the pair of electrodes being provided in a singe layer for applying an electric field to the liquid crystals, resulting in a decrease in transmission and the like.
In order to solve the problem of IPS mode liquid crystal display devices, liquid crystal display devices using an FFS mode which may be referred to as an oblique electric filed system have been developed (see JP-A-2001-235763 and JP-A-2002-182230). In the FFS mode liquid crystal display device, a pixel electrode and a common electrode for applying an electric field to a liquid crystal layer are respectively disposed in different layers with an insulator therebetween.
FFS mode liquid crystal display devices have a wider viewing angle and a higher contrast compared to IPS mode liquid crystal display devices, and are also capable of a low voltage driving and a bright display due to higher transmission. In addition, FFS mode liquid crystal display devices have a larger area in which the pixel electrode and the common electrode overlap in planar view compared to IPS mode liquid crystal display devices, further resulting in a larger retention volume. Therefore, FFS mode liquid crystal display devices are advantageous in that a storage capacitor line does not need to be additionally provided.
However, since a step is formed on the surface of the pixel electrode which overlaps with a switching element or a common line in a related-art FFS mode liquid crystal display device, liquid crystal molecules are misaligned in the step portion. Therefore, since the step portion of the related-art FFS mode liquid crystal display device is a region which substantially does not contribute to displaying an image, light shielding by a black matrix on a color filter substrate is necessary, whereby the aperture ratio deceases by a share corresponding to the step portion.
In order to eliminate the step portion, the pixel electrode or the common electrode may be disposed on a planarization film such as that used in the liquid crystal display device of the VA system or the MVA system described above. However, with this configuration, two contact holes need to be formed to electrically connect the pixel electrode and the switching element, and the common electrode and the common line, since the switching element and the common line are formed below the planarization film. The two contact holes usually cannot be formed simultaneously since the pixel electrode and the common electrode are disposed in different layers. Thus, there is a problem in that separate steps are necessary to form the contact holes.